Chemists make hard-to-catch molecules

In an advance that could lead to new catalysts for chemical reactions, scientists in Europe have found a novel way to stabilize highly reactive molecules called carbenes.

A molecular model shows the structure of a stable carbene. A central carbon (arrow) bonds on the left to an amino group and on the right to an aryl group. Science

A carbene contains a central carbon atom with two bonds, rather than carbon’s typical four. This arrangement leaves the carbon with two free electrons hungry to react.

Carbenes usually exist only fleetingly in reactions that produce substances ranging from polymers to drugs. In the past decade or so, however, chemists have made some stable carbenes by bonding the central carbon to two bulky substituents, or groups of atoms. The chosen substituents must shield the reactive carbon physically, and both also tie up the carbon’s electrons by pushing or pulling them.

The new method for making stable carbenes, reported in the June 8 Science, is simpler: Just one substituent must have electron-moving ability. The other substituent only offers physical protection, says team member Guy Bertrand of the Université Paul Sabatier in Toulouse, France.

The group found that its less-elaborate carbenes are stable for days in solution and last indefinitely in a solid-crystal form. Yet the molecules remain reactive enough to serve as catalysts. This combination, say the researchers, should broaden the potential array of carbenes available for driving reactions.

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